1. Technical Field
The invention relates to a polarizer for two different frequency bands for exciting an antenna with a parabolic reflector, comprising a waveguide section capable of carrying in each frequency band two mutually perpendicularly linearly polarized waves, wherein for each frequency band there are connected to the waveguide section separate from each other and mutually offset in the axial direction of the waveguide section two waveguides having a rectangular cross-section, wherein for the lower frequency band for each polarization direction a respective waveguide is connected directly to the waveguide section, wherein for the higher frequency band--starting at a connecting point--each of the two waveguides is subdivided into two branches with identical rectangular cross-sections, with the branches terminating at two opposing locations of the waveguide section, and wherein the locations where the branches for the two different polarization directions terminate on the waveguide section, are circumferentially offset relative to each other by 90.degree..
2. Description of the Prior Art
Polarizers are used, for example, for exciting antennae with a parabolic reflector for line of sight radio communication, satellite communication or radio location. Polarizers can be used for either exciting the reflector through a sub-reflector (for example, Cassegrain principle) or for directly illuminating the reflector. In the following, "excitation" shall denote both transmission directions of the electro-magnetic waves, i.e. transmitted as well as received waves. In polarizers of this type, two linearly polarized electro-magnetic waves of the same frequency band are guided so that their polarization directions are orthogonal to each other. The two waves therefore do not interfere. Polarizers for a single frequency band or for two different frequency bands are known in the art.
GB 2,117,980 A1 describes a polarizer for two different frequency bands. The polarizer has two regions with circular cross-sections which are the arranged one after the other and have different inside diameters. Two waveguides are connected to each of these regions. Moreover, the region with the larger inside diameter has two different inside diameters wherein the two waveguides of this region terminate in areas having different inside diameters. This polarizer is very expensive to manufacture because the two differently sized regions have to be combined individually while observing very tight tolerances.
In the known polarizer described in EP 0 096 461 B1, the waveguides for the higher frequency band are subdivided, starting at a connection point, into two branches which are terminated in the waveguide section at diametrically opposed points. The connection point is formed as a T-shaped hybrid coupler and provided with two connections. In normal operation, the respective waveguide is coupled in phase via one of the connections which is coupled via a waveguide section to the hybrid coupler. The other connection which is not in phase, is covered with a short-circuit plate. The construction of the polarizer is very costly, in particular in the region adapted for the higher frequency band, which requires two hybrid couplers with the connected waveguide and two additional connections which have to be covered, for example, by short circuit plates. These components also add to the weight, making the installation of the polarizer on the reflector of an antenna more difficult.